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17.3 PCB Signal Integrity Concerns

1. Provide resistor terminations for all high-speed switching signals and clock lines >15 MHz on the PCB. Locate

these terminations on the driver side of the trace. The exception is high speed silicon impedance controlled

dif

ferential signal,s such as USB that do not require resistor termination because the drivers themselves have

guaranteed output impedance by specification.

For bi-directional signals, place a series resistor at the output of any output driver on the bus.

Figure 17-6. High Speed Signal Termination Scope Plot Illustration

There are two methods to determine the required termination resistor values.

Step 1: Calculate the PCB line impedance by entering the PCB trace information into an impedance

calculator:

https://www

.eeweb.com/tools/microstrip-impedance = Trace Impedance

Table 17-1. PCB Trace Impedance Calculation Example

Trace Width PCB Height between layers Trace Thickness PCB Substrate Dielectric FR4 Bd Trace Impedance

0.175mm 113µm 1.41 mils (1 oz Cu) 4 54.6 Ohms

Table 17-2. Typical Trace Thicknesses Based on Copper Weight

Cu weight Thickness in Mils

1 oz 1.4 mils

2 oz 2.8 mils

3 oz 4.2 mils

4 oz 5.6 mils

Step 2: Once the trace impedance is calculated by entering the PCB trace information then:

Method 1 of 2: (Ball Park method)

PCB signal series termination resistor(s) value = {[(VDD-VOH(min)) / IOH(max)] - T

race Impedance}

= {[(3.3-2.4) /10 ma] – 54.6 Ohms}

Comprehensive PCB Layout Guidelines and Recommenda...

echnology Inc.

and its subsidiaries

Manual

DS70005439B-page 70

Microchip Technology SAM Series User Manual

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